Could you please tell us a little more information? The more information we have, the more we can provide back. And more accurately, too.
First, what feed is she on? And does she get any supplements such as oyster shell? Has her laying dropped off with out her being in a molt? Has she ever had any leathery shelled eggs or shell-less eggs? And as Ivan asked, could the yellow be yolk mixed in with her feces?
Pick her up and feel her carefully but delicately. Does her abdoment feel overly lumpy or fluid-filled to you? Check her carefully for mites and lice - do you see any, particularly around her vent? Check there as well for any white or black residue on the skin near the vent.
First a diet concern:
She should be on a laying mash as at least 90% of her diet, the other 10% being comprised of treats, grains, or just more laying feed. She should also have free choice oyster shell at all times as the laying pellets are made for the scientifically average hen, but not all "in real life" hens - some of which need more calcium. Oyster shell is a very dissolvable and absorbable form of calcium.
The complete laying feed takes care of the phosphorus and vitamin D needed to get the calcium to absorb (usually) but sometimes the D degrades. And certainly with an ill hen, you would want to supplement. her droppings are not normal and do indicate a few issues.
Addressing the sours:
First, good eye saying they look like scours. Scours are caused by a bacterial imbalance in animals. it's usually a cattle term, in chickens we call it "diarrhea" but personally I like "scours" better as it indicates what's going on inside the chicken better. You can and should always replenish the live bacteria in a chickens' gut any time they have diarrhea or scours. You can easily do this using plain yogurt in all cases except when you're medicating with a medicine that contains "mycin" or "cycline" as the last part of their name. In those cases, Probios powder or paste for livestock (from the feed store) really help.
Addressing the vent:
You should clean the vent with warm water and possibly a gentle antiseptic. Dry it well. Examine it for any sores, very common during diarrhea. Dress all sores with antiseptic ointment (neosporin, for example). If the sores are extensive, consider buying screw-worm wound spray from the feedstore and spraying on the vent after covering the vent opening itself with two gloved fingers. Flies will be attracted to the droppings and they must be removed, the vent kept dry, so that maggots don't happen. If they DO happen simply wash them off, dry and retreat the area again. They can sometimes hatch in 2 or 3 batches. Keep the droppings off of her as often as possible.
Do you have any antibiotics there, such as penicillin injectable (in case of egg binding or egg peritonitis)?
I suspect that your hen hasn't been laying, possibly because of a calcium imbalance, and that she has laid some eggs that were shell-less or with soft shells. Possibly one or two have broken in her at some point and are expelling. The remaining bits have caused her to get an infection in her cloaca, resulting in the smell - although I can't yet rule out Vent Gleet or a similar yeast infection. But yogurt will help with that as we wait for your answers.
I would make sure that this bird is eating even if you have to trick her into it. Vitamins and minerals in the water can help. If she looks droopy to you, I'd consider giving her a round of penicillin injections. But I'm waiting for your answers on that. If you do so, buy the 18 gauge needles, and 3 cc syringes. 18 gauge (the larger size) is required for penicillin which has big particles in it and is very very thick. You can buy Penicillin G Procaine or regular penicillin.. If neither is available you could buy fish penicillin but giving it orally to a bird who already has diarrhea might be too much strain.
I'll wait for your answers before pinning down one of my suggestions, but wanted you to be prepared in case you go shopping today.
By the way, it should be obvious, but please do isolate her into a small area, free of summer's heat, with food and water free choice so that you can monitor her droppings etc, and so she won't have to walk for feed.
I am emailing you this information as well.